NMR logging interpretation of solid invasion

ABSTRACT

A method for estimating an effect on nuclear magnetic resonance (NMR) measurements of an invasion of solid particles into pores of an earth formation penetrated by a borehole includes conveying a carrier through the borehole and performing an NMR measurement on a volume of interest in the formation to provide a relaxation time constant using an NMR tool disposed at the carrier. The method further includes receiving information describing the solid particles in the pores and quantifying, using a processor, an effect on the measured relaxation time constant due to the invasion of solid particles using the received information.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/081,218 filed Nov. 15, 2013, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

Geologic formations are used for many applications such as hydrocarbonproduction, geothermal production, and carbon dioxide sequestration.Typically, boreholes are drilled into the formations to provide accessto them. Various downhole tools may be conveyed in the boreholes inorder to characterize the formations. Characterization of the formationsand the fluids within provides valuable information related to theintended use of the formation so that drilling and production resourcescan be used efficiently.

One type of downhole tool is a nuclear magnetic resonance (NMR) toolthat measures the nuclear magnetic properties of formation materials.NMR logging data of unconsolidated sands suggests that solid invasioninto the formation may occur. Solid invasion is the phenomenon of fineparticles in the drilling mud invading into the formation before orduring the forming of mud cake. This phenomenon may also occur forcarbonate reservoir. Fine carbonate particles produced from thedrilling, especially when a polycrystalline diamond compact (PDC) bit isused, mixes into the mud and may then invade into the formation for someor many carbonate wells with favorable porosity and permeabilitycondition. Because solid invasion can affect NMR measurements offormation materials of interest, it would be well received in thedrilling and geophysical exploration industries if the effects of solidinvasion could be quantified.

BRIEF SUMMARY

Disclosed is a method for estimating an effect on nuclear magneticresonance (NMR) measurements of an invasion of solid particles intopores of an earth formation penetrated by a borehole. The methodincludes: conveying a carrier through the borehole; performing an NMRmeasurement on a volume of interest in the formation to provide arelaxation time constant using an NMR tool disposed at the carrier;receiving information describing the solid particles in the pores; andquantifying, using a processor, an effect on the measured relaxationtime constant due to the invasion of solid particles using the receivedinformation.

Also disclosed is an apparatus for estimating an effect on nuclearmagnetic resonance (NMR) measurements of an invasion of solid particlesinto pores of an earth formation penetrated by a borehole, the apparatuscomprising: a carrier configured to be conveyed through the borehole; aNMR tool disposed at the carrier and configured to perform a NMRmeasurement on a volume of interest in the formation to provide arelaxation time constant; and a processor. The processor is configuredto receive information describing the solid particles in the pores andquantify an effect on the measured relaxation time constant due to theinvasion of solid particles using the received information.

BRIEF DESCRIPTION OF THE DRAWINGS

The following descriptions should not be considered limiting in any way.With reference to the accompanying drawings, like elements are numberedalike:

FIG. 1 illustrates a cross-sectional view of an exemplary embodiment ofa nuclear magnetic resonance (NMR) tool disposed in a boreholepenetrating the earth;

FIG. 2 depicts aspects of a pore invaded with fine solids to form auniform suspension;

FIGS. 3A, 3B, and 3C, collectively referred to as FIG. 3, depict aspects

FIG. 4 is a flow chart of a method estimating an effect on nuclearmagnetic resonance (NMR) measurements of an invasion of solid particlesinto pores of an earth formation penetrated by a borehole.

DETAILED DESCRIPTION

A detailed description of one or more embodiments of the disclosedapparatus and method presented herein by way of exemplification and notlimitation with reference to the figures.

Disclosed are apparatus and method estimating an effect on nuclearmagnetic resonance (NMR) measurements of an invasion of particles intopores of an earth formation. The effects are quantified so that theoutput of an NMR tool can be corrected or uncorrected NMR tool outputcan be compensated for during interpretation by an analyst.

FIG. 1 illustrates a cross-sectional view of an exemplary embodiment ofan NMR tool 10 disposed in a borehole 2 penetrating the earth 3, whichincludes an earth formation 4. The NMR tool 10 is configured to performNMR measurements on the formation 4. The NMR measurements yieldtransverse relaxation times T₂, which are exponential decay timeconstants that correspond to a characteristic or property of theformation 4 material. Transverse relaxation relates to the loss ofcoherent energy by protons in the formation 4 material while precessingabout a static magnetic field during an NMR measurement. There is notone single value of T₂ for formation rock but a wide distribution ofvalues lying anywhere between fractions of a millisecond and severalseconds for example. The distribution of T₂ values is the principaloutput of the NMR tool 10 and may be referred to as an NMR log.Components in the NMR tool 10 includes a static magnetic field source 13that magnetizes formation materials and an antenna 14 that transmitsprecisely timed bursts of radio-frequency energy that provides anoscillating magnetic field. In a time period between these pulses, theantenna receives a decaying echo signal from those hydrogen protons thatare in resonance with the static magnetic field produced by the staticmagnetic field source. NMR measurements are performed in a cylindricalvolume surrounding the NMR tool 10 referred to as a volume of interest9. Because a linear relationship exists between the hydrogen protonresonance frequency and the strength of the static magnetic field, thefrequency of transmitted radio-frequency energy can be tuned toinvestigate volumes of interest having different diameters around theNMR tool 10. It can be appreciated that the NMR tool 10 may include avariety of components and configurations as known in the art of NMR. Inthat NMR tools are known in the art, specific details of components andconfigurations of these tools are not discussed in further detail.

The NMR tool 10 is conveyed through the borehole 2 by a carrier 5, whichcan be a drill tubular such as a drill string 6. A drill bit 7 isdisposed at the distal end of the drill string 6. A drill rig 8 isconfigured to conduct drilling operations such as rotating the drillstring 6 and thus the drill bit 7 in order to drill the borehole 2. Inaddition, the drill rig 8 is configured to pump drilling mud (i.e.,drilling fluid 16) through the drill string 6 in order to lubricate thedrill bit 7 and flush cuttings from the borehole 2. Downhole electronics11 are configured to operate the NMR tool 10, process measurement dataobtained downhole, and/or act as an interface with telemetry 17 tocommunicate data 18 or commands 19 between downhole components and acomputer processing system 12 disposed at the surface of the earth 3.Non-limiting embodiments of the telemetry include pulsed-mud and wireddrill pipe for real time communications. System operation and dataprocessing operations may be performed by the downhole electronics 11,the computer processing system 12, or a combination thereof. In analternative embodiment, the carrier 5 may be an armored wireline, whichmay also provide communications with the surface processing system 12.

The effects of solid invasion on NMR logging can be twofold: (1) theporosity is underestimated because solids occupy part of the pore spaceand (2) the NMR T₂ and T₁ relaxation time constants are shorter thanwithout the solids invasion. As discussed below, solid invasion canaffect NMR log measurements significantly when it occurs.

When there is no solid invasion, the NMR T₂ relaxation time constant maybe represented as follows:

$\begin{matrix}{\frac{1}{T_{2}} = {\frac{1}{T_{2}^{bulk}} + \frac{1}{T_{2}^{diff}} + \frac{1}{T_{2}^{surf}}}} & (1)\end{matrix}$where T₂ is the overall transverse relaxation time constant, T₂ ^(bulk)is the bulk relaxation time constant, T₂ ^(diff) is the diffusionrelaxation time constant, and T₂ ^(surf) is the surface relaxation timeconstant. The T₁ relaxation time may be represented as follows:

$\begin{matrix}{\frac{1}{T_{1}} = {\frac{1}{T_{1}^{bulk}} + \frac{1}{T_{1}^{surf}}}} & (2)\end{matrix}$

where T₁ is the overall longitudinal relaxation time constant, T₁^(bulk) is the bulk relaxation time constant, and T₁ ^(surf) is thesurface relaxation time constant. The diffusion relaxation time does notapply for T₁.

T₂ ^(surf) may be expressed in the following relationship:

$\begin{matrix}{\frac{1}{T_{2}^{surf}} = {\rho\frac{A}{V}}} & (3)\end{matrix}$

where ρ, A, and V are the surface relaxivity, pore surface area, andpore volume, respectively. The corresponding equation for T₁ is:

$\begin{matrix}{\frac{1}{T_{1}^{surf}} = {\rho_{1}\frac{A}{V}}} & \left( 3^{\prime} \right)\end{matrix}$where ρ₁ may be different from ρ.

In general, solid invasion will not influence the bulk relaxation timeconstant T₂ ^(bulk), but it does have an influence on the diffusionrelaxation time constant T₂ ^(diff) which may be smaller due to thedecreasing of effective porosity and increasing of tortuosity caused bythe invaded particles. However, this influence is generally smallcompared to the surface relaxation time constant T₂ ^(surf). Thus, T₂^(diff) is neglected in the following. Also since both T₂ and T₁ are nowgoverned by the same relaxation mechanisms (i.e. bulk and surfacerelaxation), the teachings continue by only referring to T₂ in thefollowing, although all reasoning applies to T₁ as well.

It is assumed that (a) the formation has porosity ϕ (i.e., ratio ofvolume of void space in formation rock to the total volume of the rock),(b) the invading fluid (i.e., drilling mud) having solid particlesdominates the formation fluid, and (c) the drilling mud and theformation fluid are miscible (i.e., water zone for water based mud oroil zone for oil based mud). If the drilling mud and the formation fluidare not miscible, then solid invasion will have little or no effects onthe majority of the fluid in the formation due to limited or no invasionfluid to solid surface contact.

Referring to FIG. 2, a homogeneous suspension of solid invasionparticles 21 in a pore 20 in the formation 4 is illustrated after solidinvasion. The pore 20 is just one of many pores in the formation 4.Assuming the total volume of all the solid invasion particles isrepresented as ϕ_(s) (expressed in porosity units) and each of theparticles is spherical with radius r, then ϕ_(s) may be calculated asfollows:ϕ_(s) =ϕ·n·4/3πr ³   (4)where n is the number density or number of particles in a unit porevolume.

The porosity measured from the NMR log (ϕ_(log)) is expressed inequation (5).ϕ_(log)=ϕ−ϕ_(s)=(1−4/3nπr ³)   (5)

The solid invasion particles contribute a surface relaxation mechanismto the fluid in the pore. Considering that the volume of the poreillustrated in FIG. 2 is V, then the number of solid invasion particlesN in that pore is expressed in equation (6).N=n·V   (6)

It follows that the total surface area of all the solid invasionparticles in the pore (A_(s)) can be expressed using equation (7).A _(s)=(Vn)·4πr ²   (7)

Further, it follows that the total volume of the fluid in the pore(V_(f)) is the total void space or volume of the pore minus the totalvolume of all the solid invasion particles in the pore as expressed inequation (8).V _(f) =V(1−4/3nπr ³)   (8)

Finally, considering that the invasion solid particles may havedifferent surface relaxivity from that of the pore surface, the surfacerelaxation time constant term in equation (2) for fluid within the poremay be expanded as follows:

$\begin{matrix}{\frac{1}{T_{2,{SI}}^{surf}} = {{\rho\frac{A}{V_{f}}} + {\rho_{s}\frac{A_{s}}{V_{f}}}}} & (9)\end{matrix}$where the subscript SI indicates surface relaxation T₂ with invasion ofsolid particles and ρ_(s) is the surface relaxivity of the solidinvasion particles. Equation (9) shows that solids invasion modifies thefluid relaxation time in two ways. First, it enhances the pore surfacerelaxation contribution by decreasing the fluid volume from V to V_(f)while exposing to the same pore surface area A, as indicated by thefirst term on the right side. Second, the invasion solid itselfcontributes a surface relaxation mechanism as shown in the second termon the right in Eq. (9).

Equations (5), (7) and (8) may be inserted into equation (9) to deriveequation (10).

$\begin{matrix}{\frac{1}{T_{2,{SI}}^{surf}} = {\left( {\frac{1}{T_{2}^{surf}} + {\rho_{s}n\; 4\pi\; r^{2}}} \right)\frac{\phi}{\phi_{\log}}}} & (10)\end{matrix}$

Using the relationship in equation (5), the density n can be removedfrom equation (10) to arrive at equation (11).

$\begin{matrix}{\frac{1}{T_{2,{SI}}^{surf}} = {\left( {\frac{1}{T_{2}^{surf}} + {\rho_{s}\frac{3}{r}\frac{\phi_{s}}{\phi}}} \right)\frac{\phi}{\phi_{\log}}}} & (11)\end{matrix}$

Equation (11) can be rewritten as equation (12) where R_(SI)=ϕ_(s)/ϕ,the ratio of invaded porosity to the total porosity.

$\begin{matrix}{\frac{1}{T_{2,{SI}}^{surf}} = {{\frac{1}{T_{2}^{surf}}\frac{1}{1 - R_{SI}}} + {\rho_{s}\frac{3}{r}\frac{R_{SI}}{1 - R_{SI}}}}} & (12)\end{matrix}$

Note that the factor 3/r represents the surface area-to-volume ratioA_(s)/V_(s) of the solids and depends on their shape. While a sphereresults in a surface-to-volume ratio of 3/r, a cube will yield 6/α ortetrahedron 6*sqrt(6)/α where a represents the side length. Depending onthe shape of the solids, the surface-to-volume ratio has to be adjustedaccordingly.

Next, some special cases are considered. In case 1, the solid invasionparticles have minimum or negligible (e.g., ≤5%) surface relaxationcompared to that of the formation fluid in the pores. For example,consider carbonate reservoir having an oil zone and the solid invasionparticles are carbonate powder freshly ground by the drilling bits.These freshly ground carbonate “particles” are water wet on the surfaceand, therefore, have minimum contribution to the relaxation of oil inthe pores. Thus, equation (10) may be simplified to equation (13).

$\begin{matrix}{T_{2,{SI}}^{surf} = {T_{2}^{surf}\frac{\phi_{\log}}{\phi}}} & (13)\end{matrix}$

Eq. (13) indicates in this special case that the measured T₂ is directlyproportional to the ratio of remaining pore space to the original porespace. For example, if solid invasion particles occupy 20% of the porespace, the remaining porosity is 80% of the real porosity; the measuredT₂ is reduced to 80% of its original value as well. An interestingderivative is that if these solid particles are flushed out later timeand relog is followed, the NMR measurements will indicate a largerporosity and the measured NMR T₂ will also be longer.

In case 2, the condition of the solid invasion particles dominating themeasured fluid relaxation time is considered. That is, bulk relaxationand pore surface relaxation are negligible (e.g., ≤5%) compared to thesurface relaxation of the solid particles. From Eq. (11), invaded solidsdominate the relaxation of pore fluid when equation (14) is satisfied.In one or more embodiments, the symbol “>>” relates to the lesser valuebeing 10% or less of the greater value.

$\begin{matrix}{{\rho_{s}\frac{3}{r}\frac{\phi_{s}}{\phi - \phi_{s}}}\operatorname{>>}\frac{1}{T_{2}^{surf}}} & (14)\end{matrix}$

Considering light oil or water in large pores, then the right side ofequation (14) is equal to or smaller than one. Accordingly, equation(14) then requires the relationship in equation (15).

$\begin{matrix}{{\frac{\rho_{s}}{r}\frac{1}{\left( {\phi/\phi_{s}} \right) - 1}}\operatorname{>>}\frac{1}{3}} & (15)\end{matrix}$

For sandstone in general, ρ_(s)˜1.6×10⁻⁵ m/s; and a 20% solid invasionwould only require r<<1.2×10⁻⁵ m to satisfy equation (14). A 5% solidinvasion with particle size 0.1 m will also approximately satisfyequation (14). Apparently this condition in the field can be easilysatisfied. Furthermore, if the invaded solid particles have paramagneticminerals with a much larger surface ρ_(s), then a solid invasion of lessthan 1%, which does not have a measurable effects on the porosity, canstill have a significant influence on the transverse relaxation timeconstant (T₂) and also the longitudinal relaxation time constant (T₁) ofpore fluids.

FIG. 3 depicts aspects of the surface relaxation time constant due toinvaded solids in the fluids in the pores according to the particleradius calculated for different surface relaxivity values. In FIG. 3A,the solid invasion particles are carbonate and ρ_(s)=0.5×10⁻⁵ m/s. InFIG. 3B, the solid invasion particles are sandstone and ρ_(s)=1.6×10⁻⁵m/s. And, in FIG. 3C, the solid invasion particles are rock containingparamagnetic minerals and ρ_(s)=1.6×10⁻⁴ m/s.

It can be appreciated that by using the above equations, a processor mayprovide compensation for solid invasion by outputting a true value of aproperty of interest. What is wanted is the distribution of transverserelaxation time constants T₂'s that relate to the pores in the formationthat are considerably away from the borehole and, thus, do not havesolid invasion. From the T₂'s of the pores without solid invasion, theactual porosity of the formation may be determined. However, the solidinvasion particles distort or modify the measured T₂'s affecting thedetermination of the true properties of the formation away from theborehole. As shown in equation (1), the measured T₂ is affected by threerelaxation components—bulk, diffusion, and surface. Depending on thecomposition and/or physical properties of the solid invasion particles,the surface relaxation component may be affected the most by the solidinvasion particles. The surface relaxation component affected by solidinvasion is referred to as T_(2,SI) ^(surf). Equations (10)-(12) relateT_(2,SI) ^(surf) to T₂ ^(surf) which is the surface relaxation componentif solid invasion particles were not present in the pores. In general,other variables are required to determine the relationship betweenT_(2,SI) ^(surf) and T₂ ^(surf) using equations (12)-(11) such as theradius or average radius of the solid invasion particles, the surfacerelaxivity of those particles, and the ratio of porosity occupied bysolids to the total porosity of the pores being evaluated by the NMRtool 10 for example. Values of these other variables may be obtainedfrom previously obtained data, from laboratory analysis of the actualparticles downhole during the NMR measurements, laboratory analysis ofparticles similar to those particles expected downhole, and measurementsusing other types of downhole tools such as neutron tools for example.Once T_(2,SI) ^(surf) is quantified, its effect on the measured T's canalso be quantified using equation (1) for example. By removing theinfluence of T_(2,SI) ^(surf) from the measured T₂'s, output from theNMR tool 10 can be compensated, by a processor for example, to providethe true T₂'s that relate to pores deeper in the formation that do nothave solid invasion. In one or more embodiments, the influence ofT_(2,SI) ^(surf) is removed by subtracting its inverse from the rightside of equation (1) as the magnitude of the inverse of T_(2,SI) ^(surf)is generally much greater than the inverse of T₂ ^(surf). Alternatively,if uncompensated NMR tool output is being analyzed, then thequantification can be used to help interpret the uncompensated NMR dataobtained from an NMR tool such as by applying a correction that wouldnegate the influence of the solid invasion particles.

It can be appreciated that porosity is just one property that may bedetermined using the transverse relaxation time constants that arecompensated for invasion of solid particles in pores near the boreholecontaining the NMR tool. The techniques disclosed herein may also beused to improve the accuracy in determining other formation propertiesfrom distributions of measured transverse and/or longitudinal relaxationtime constants. Other properties may include reservoir permeability andhydrocarbon typing, which includes differentiating between differenttypes of fluids and identifying a type of hydrocarbon present in theformation. These other properties may be determined using knownrelationships that relate a property of interest to a distribution ofrelaxation time constants.

FIG. 4 is a flow chart for a method 40 for estimating an effect onnuclear magnetic resonance (NMR) measurements of an invasion of solidparticles into pores of an earth formation penetrated by a borehole.Block 41 calls for conveying a carrier through the borehole. Block 42calls for performing an NMR measurement on a volume of interest in theformation to provide a relaxation time constant using an NMR tooldisposed at (i.e., in or on) the carrier. The relaxation time constantmay include a transverse relaxation time constant and/or a longitudinaltime constant. Block 43 calls for receiving information describing thesolid particles in the pores. The information may be received using aprocessor. The received information may include at least one of a sizeof each of the particles, a surface relaxivity of each of the particles,an amount of space the particles occupy in each of the pores, and asurface area-to-volume ratio of the particles. This information may bepre-known or it may be calculated from other information or data. Block44 calls for quantifying, using a processor, an effect on the measuredtransverse relaxation time constant due to the invasion of solidparticles using the received information. The method 40 may also includecalculating the transverse relaxation time constant T₂'s for pores awayfrom the borehole not having solid invasion using the T₂'s measured bythe NMR tool and the transverse surface relaxation time constantT_(2,SI) ^(surf). The method 40 may also include displaying thequantified effect or the calculated T₂'s for pores away from theborehole not having solid invasion using a user interface such as adisplay as the one in the computer processing system 12. The method 40may also include using the quantified effect to improve the accuracy indetermining a formation property for pores not having solid invasionaway from the borehole. By knowing the quantified effect, the quantifiedeffect can be compensated for in processes for estimating a formationproperty using relaxation time constants obtained by an NMR tool probinga formation. Non-limiting examples of formation properties estimatedfrom relaxation time constants include porosity, permeability, andhydrocarbon typing. In that processes for estimating formationproperties using relaxation time constants are known in the art, theseprocesses are not discussed in further detail.

In support of the teachings herein, various analysis components may beused, including a digital and/or an analog system. For example, thedownhole electronics 11 or the computer processing system 12 may includedigital and/or analog systems. The system may have components such as aprocessor, storage media, memory, input, output, communications link(wired, wireless, pulsed mud, optical or other), user interfaces (e.g.,a display), software programs, signal processors (digital or analog) andother such components (such as resistors, capacitors, inductors andothers) to provide for operation and analyses of the apparatus andmethods disclosed herein in any of several manners well-appreciated inthe art. It is considered that these teachings may be, but need not be,implemented in conjunction with a set of computer executableinstructions stored on a non-transitory computer readable medium,including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks,hard drives), or any other type that when executed causes a computer toimplement the method of the present invention. These instructions mayprovide for equipment operation, control, data collection and analysisand other functions deemed relevant by a system designer, owner, user orother such personnel, in addition to the functions described in thisdisclosure.

Further, various other components may be included and called upon forproviding for aspects of the teachings herein. For example, a powersupply (e.g., at least one of a generator, a remote supply and abattery), cooling component, heating component, magnet, electromagnet,sensor, electrode, transmitter, receiver, transceiver, antenna,controller, optical unit, electrical unit or electromechanical unit maybe included in support of the various aspects discussed herein or insupport of other functions beyond this disclosure.

The term “carrier” as used herein means any device, device component,combination of devices, media and/or member that may be used to convey,house, support or otherwise facilitate the use of another device, devicecomponent, combination of devices, media and/or member. Other exemplarynon-limiting carriers include drill strings of the coiled tube type, ofthe jointed pipe type and any combination or portion thereof. Othercarrier examples include casing pipes, wirelines, wireline sondes,slickline sondes, drop shots, bottom-hole-assemblies, drill stringinserts, modules, internal housings and substrate portions thereof.

Elements of the embodiments have been introduced with either thearticles “a” or “an.” The articles are intended to mean that there areone or more of the elements. The terms “including” and “having” areintended to be inclusive such that there may be additional elementsother than the elements listed. The conjunction “or” when used with alist of at least two terms is intended to mean any term or combinationof terms. The term “coupled” relates to one component being coupled toanother component either directly or indirectly via an intermediatecomponent.

While one or more embodiments have been shown and described,modifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustrations and not limitation.

It will be recognized that the various components or technologies mayprovide certain necessary or beneficial functionality or features.Accordingly, these functions and features as may be needed in support ofthe appended claims and variations thereof, are recognized as beinginherently included as a part of the teachings herein and a part of theinvention disclosed.

While the invention has been described with reference to exemplaryembodiments, it will be understood that various changes may be made andequivalents may be substituted for elements thereof without departingfrom the scope of the invention. In addition, many modifications will beappreciated to adapt a particular instrument, situation or material tothe teachings of the invention without departing from the essentialscope thereof. Therefore, it is intended that the invention not belimited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A method for estimating an effect on nuclearmagnetic resonance (NMR) measurements of an invasion of solid particlesinto pores of an earth formation penetrated by a borehole, the methodcomprising: conveying a carrier through the borehole; performing an NMRmeasurement on a volume of interest in the formation to provide arelaxation time constant using an NMR tool disposed at the carrier;receiving information describing the solid particles in the pores, theinformation comprising a shape property of the solid particles;quantifying, using a processor, the effect on the measured relaxationtime constant due to the invasion of the solid particles using thereceived information; correcting the NMR measurement by the processorusing the quantified effect to provide corrected NMR output; andoperating drilling or production resources in response to a parameter ofthe earth formation determined from the corrected NMR output having avalue that provides input for operating the drilling or productionresources.
 2. The method according to claim 1, wherein the solidparticles come from drilling fluid used to drill the borehole.
 3. Themethod according to claim 1, wherein the relaxation time constantcomprises at least one of a transverse relaxation time constant and alongitudinal relaxation time constant.
 4. The method according to claim1, wherein the information comprises at least one selection from a groupconsisting of a size of each of the particles, a surface relaxivity ofeach of the particles, and an amount of space the particles occupy ineach of the pores.
 5. The method according to claim 1, wherein the solidinvasion particles have negligible surface relaxation compared to thesurface relaxation of formation fluid in the pores.
 6. The methodaccording to claim 5, wherein quantifying comprises using the followingrelationship:$T_{2,{SI}}^{surf} = {T_{2}^{surf}\frac{\phi_{\log}}{\phi}}$ whereT_(2,SI) ^(surf) is the surface transverse relaxation time constant withsolid invasion, T₂ ^(surf) is the surface transverse relaxation timeconstant without solid invasion, ϕ_(log) is the porosity determined fromthe NMR measurement, and ϕ is the actual porosity of formation.
 7. Themethod according to claim 1, wherein bulk relaxation and pore surfacerelaxation are negligible compared to the surface relaxation of thesolid particles.
 8. The method according to claim 7, further comprisingcalculating a surface relaxation time constant for pores having theinvasion of solid particles.
 9. The method according to claim 8, whereincalculating a surface relaxation time constant for pores comprises usingthe following relationship:$\frac{1}{T_{2,{SI}}^{surf}} = {\rho_{s}\frac{A_{s}}{V_{s}}\frac{\phi_{s}}{\phi_{\log}}}$where T_(2,SI) ^(surf) is the surface transverse relaxation timeconstant of the invaded solid particles, ϕ_(1og) is the porositydetermined from the NMR measurement, ϕ_(s), is the porosity occupied bythe solid particles, ρ_(s) is the surface relaxivity of the solidinvasion particles, and A_(s)/V_(s) represents the surfacearea-to-volume ratio of the solid particles.
 10. The method according toclaim 9, wherein at least a portion of the solid particles are modeledas a sphere and A_(s)/V_(s) is 3/r where r is the radius of the sphere.11. The method according to claim 1, further comprising using thequantified effect to improve the accuracy in determining a formationproperty for pores not having solid invasion away from the borehole. 12.The method according to claim 11, wherein the formation property is atleast one of porosity, permeability, and hydrocarbon typing.
 13. Anapparatus for estimating an effect on nuclear magnetic resonance (NMR)measurements of an invasion of solid particles into pores of an earthformation penetrated by a borehole, the apparatus comprising: a carrierconfigured to be conveyed through the borehole; a NMR tool disposed atthe carrier and configured to perform a NMR measurement on a volume ofinterest in the formation to provide a relaxation time constant; and aprocessor configured to: receive information describing the solidparticles in the pores, the information comprising a shape property ofthe solid particles; quantify an effect on the measured relaxation timeconstant due to the invasion of solid particles using the receivedinformation; correct NMR measurements using the quantified effect toprovide corrected NMR output; a drilling or production resourceconfigured to be operated in response to a parameter of the earthformation determined from the corrected NMR output having a value thatprovides input for operating the drilling or production resources. 14.The apparatus according to claim 13, wherein the solid particles comefrom drilling fluid used to drill the borehole.
 15. The apparatusaccording to claim 13, wherein the relaxation time constant comprises atleast one of a transverse relaxation time constant and a longitudinalrelaxation time constant.
 16. The apparatus according to claim 13,wherein the processor is further configured to calculate a surfacetransverse relaxation time constant T_(2,S) for pores with invasion ofsolid particles using T_(2,SI) ^(surf), where T_(2,SI) ^(surf) is thesurface transverse relaxation time constant of the invaded solidparticles.
 17. The apparatus according to claim 16, wherein theprocessor is further configured to calculate a transverse relaxationtime constant T₂ for pores without solid invasion away from the boreholeusing T_(2,SI) ^(surf).
 18. The apparatus according to claim 13, whereinthe processor is further configured to estimate a formation propertywith improved accuracy using the quantified effect.